JP2008018448A - Method of continuous casting - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of continuous casting, which method can improve the life of the whole casting mold by elongating the life of the upper side of the casting mold. <P>SOLUTION: In the continuous casting method, molten steel 2 existing in a tundish is supplied into the casting mold 4, and the supplied molten steel 2 is cast while being electromagnetically stirred in one direction. When the molten steel 2 is supplied again into the casting mold 4 from the tundish after stopping the supply of the molten steel 2 to the casting mold 4 from the tundish, the casting is carried out after changing the level of the molten steel 2 in the casting mold 4 by 15 mm or more than the level of the molten steel 2 before stopping the supply of the molten steel 2. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a continuous casting method.

Conventionally, there is a continuous casting method in which molten steel in a tundish is supplied to a mold, and the molten steel supplied to the mold is cast while electromagnetically stirring in one direction (for example, Patent Documents 1 and 2).
The mold used for this continuous casting method is made of copper-based material. Normally, when casting is repeated, the inner surface of the mold lowers the mold underneath the mold. Corrosion occurs, and a dent is formed on the lower side of the inner surface. Further, when casting is repeated, the upper side of the inner surface of the mold is also eroded by molten steel or the like, and a dent is also formed on the upper side of the inner surface.
In this way, a dent is formed on the inner surface of the mold, and when this dent becomes large, mold powder interposed between the mold and molten steel enters the dent, and unevenness of mold powder between the mold and molten steel occurs. appear. If mold powder non-uniformity occurs, the amount of heat removed from the mold will change, causing the problem of non-uniform solidification of the solidified shell. It is strongly desired to suppress it.

For the lower side of the inner surface of the mold, in order to make the lower side of the inner surface less likely to be corroded, coating (plating) such as a Co-based alloy or Ni alloy is applied to delay the formation of the dent, and thus the life of the mold The extension is planned.
Also, in order to prevent the upper surface of the inner surface of the mold from being eroded, a coating (plating) such as a Co-based alloy or Ni alloy is applied to delay the formation of the dent and extend the life of the mold. It is possible to plan.
JP-A-8-281402 JP 2002-283023 A

However, the temperature on the upper side of the mold is higher than that on the lower side of the mold, and even if the upper side of the inner surface of the mold is plated, the plating is difficult to expand and contract compared to copper, so the plating is easily peeled off from the copper. The actual situation is that only a thin plating can be applied to the upper side of the inner surface of the mold. Even if a thin plating is applied to the upper surface of the inner surface of the mold, the life of the mold cannot be made very long.
Particularly in recent years, slaps such as automobiles containing Zn have been increasingly used as a part of raw material (iron source) of molten steel.

Copper is easily corroded by Zn in the molten steel, and it is not possible to prevent this erosion simply by applying a thin plating on the inner surface of the mold. There is a problem that can not be.
FIG. 7 summarizes the results of investigations on the influence of molten steel containing Zn on the mold life when molten steel containing Zn was cast using a mold. That is, FIG. 7 shows an experiment in which continuous casting is repeatedly performed using the mold without plating on the upper inner surface of the mold, and the mold life (mold life) and the average concentration of Zn in the molten steel are calculated. It is a summary.

As shown in FIG. 7, it can be seen that the mold life is shortened as the average concentration of Zn in the molten steel 2 increases. Although a clear mechanism has not been clarified, when the average concentration of Zn increases, a metal compound (Cu—Zn alloy) is likely to be formed, and the melting point is low. Therefore, it is considered that the higher the Zn concentration, the more easily the inner surface is recessed and the life of the mold 4 is shortened.
Therefore, an object of the present invention is to provide a continuous casting method capable of improving the life of the entire mold by making it difficult to form a dent on the upper surface of the inner surface of the mold.

In order to achieve the above object, the present invention has taken the following measures.
That is, a continuous casting method in which molten steel in a tundish is supplied to a mold, and the supplied molten steel is cast while electromagnetically stirring in one direction, and after the supply of molten steel from the tundish to the mold is stopped, When the molten steel is supplied again from the tundish to the mold and cast, the molten metal level in the molten steel in the mold is changed by 15 mm or more from the molten metal level before the molten steel supply is stopped. It is in.
The inventor conducted various investigations on the inner surface of the mold after casting the molten steel. As a result, when focusing on the upper side of the inner surface of the mold, it was found that the degree of corrosion and erosion was high over a width of about 15 mm in the casting direction, particularly in the inner surface portion where the molten steel surface contacted.

Therefore, the inventor changes the position of the meniscus (molten surface level) of the molten steel, that is, changes the portion where the molten steel surface and the upper surface of the mold are in contact with each other, thereby causing corrosion and erosion on the upper surface of the mold. As a result, the lifetime of the upper side of the mold was increased.
In the continuous casting method, after the supply of molten steel from the tundish to the mold is stopped, when the molten steel is supplied again from the tundish to the mold and casting is performed (hereinafter sometimes referred to as switching), the surface level is changed. The inventor decided to change the hot water level at the time of switching.

If the molten metal level is changed at the time of switching, there is no possibility of adversely affecting the cast slab as compared to changing the molten metal level during casting. In other words, if the level of the molten metal is changed during casting, the occurrence of entrainment of the mold powderer introduced into the mold at the changed timing or the molten steel flow in the mold will change abruptly, which will adversely affect the product. However, such a situation can be prevented if the hot water level is changed at the time of switching.
Another means of the present invention is a continuous casting method in which molten steel in a tundish is supplied to a mold, and the supplied molten steel is cast while electromagnetically stirring in one direction, wherein the molten steel from the tundish to the mold is cast. When the molten steel is supplied again from the tundish to the mold after the supply is stopped, the casting is performed after changing the stirring direction for electromagnetically stirring the molten steel in the mold.

Furthermore, the inventor has advanced various investigations on the inner surface of the mold after casting molten steel. As a result, when attention was paid to the upper side of the inner surface of the mold, there was a portion where corrosion and erosion progressed locally. Normally, the mold has a rectangular shape in plan view, and when the molten steel is electromagnetically stirred, the molten steel accelerated in the horizontal direction hits the inner surface of the mold and is stirred while changing the flow direction of the molten steel. The inner surface portion where the molten steel easily hits the mold is locally eroded.
Therefore, the inventor decided to disperse corrosion and erosion on the upper surface of the inner surface of the mold by changing the stirring direction in which the molten steel in the mold is magnetically stirred to change the inner surface portion that is likely to hit the mold by electromagnetic stirring.

In general, electromagnetic stirring is to apply a magnetic field to unsolidified molten steel in a mold to give a horizontal flow, and inclusions and bubbles trapped on the growth surface of the solidified shell by the molten steel flow. Has the effect of washing away. Therefore, in electromagnetic stirring, it is preferable to secure the flow rate of the molten steel while ensuring the flow rate of the molten steel above a certain flow rate.
If the agitation direction is changed during casting, the flow of molten steel is reversed (reversed), the flow velocity becomes less than the required flow velocity, the flow is disturbed, and the action of washing out inclusions and bubbles is reduced. Since a defect may occur and this may cause a quality problem, the timing for changing the stirring direction of electromagnetic stirring is determined at the time of switching.

Another means of the present invention is a continuous casting method in which molten steel in a tundish is supplied to a mold, and the supplied molten steel is cast while electromagnetically stirring in one direction, wherein the molten steel from the tundish to the mold is cast. When the molten steel is supplied again from the tundish to the mold after the supply is stopped, the molten metal level in the molten steel in the mold is changed by 15 mm or more from the molten metal level before the molten steel supply is stopped. At the same time, casting is performed after changing the stirring direction in which the molten steel in the mold is electromagnetically stirred.
According to this, since both the change of the molten metal surface level and the change of the stirring direction are performed, erosion and corrosion on the inner surface of the mold can be more dispersed, and as a result, the life of the mold is greatly extended. be able to.

The timing at which the supply of molten steel is stopped is preferably when the type of steel to be cast is changed or when tundish maintenance is performed.
In the continuous casting method, the supply of molten steel to the mold is stopped by closing the slide valve or stopper on the immersion nozzle provided in the tundish, and then the sequence block is put into the mold and the slide valve or stopper is turned on. By opening, slabs of different steel types are continuously cast.
Also, in the continuous casting method, refractory material provided in the tundish will melt and casting will repeat, and slag will accumulate in the tundish. The tundish refractories are exchanged and maintenance is performed to remove slag in the tundish.

  Therefore, when changing the type of steel to be cast or when performing maintenance on the tundish, the supply of molten steel to the mold is always stopped. If the stirring direction is changed, continuous casting can be performed efficiently.

  According to the present invention, the lifetime of the entire mold is improved by making it difficult to form a recess on the upper surface of the inner surface of the mold.

The continuous casting method of the present invention will be described.
FIG. 1 shows a continuous casting apparatus that performs casting by the continuous casting method of the present invention. However, this invention is not limited to what uses this installation.
As shown in FIG. 1, the continuous casting apparatus 1 is a bloom continuous casting apparatus or a billet continuous casting apparatus, to which a tundish 3 for temporarily storing molten steel 2 and molten steel 2 from the tundish 3 are supplied. And a plurality of support rolls 5 for pulling out a slab formed by the mold 4 and supporting the slab. An electromagnetic stirring device (M-EMS) 6 for electromagnetically stirring the molten steel 2 in the mold 4 is disposed outside the mold 4.

The tundish 3 has a bottomed box shape as a whole, and an immersion nozzle 7 is provided at the bottom of the tundish 3. The immersion nozzle 7 can be opened and closed by a slide valve 8, and the injection of the molten steel 2 into the mold 4 by the tundish 3 can be stopped or restarted by opening and closing the immersion nozzle 7.
The mold 4 is formed in a rectangular tube shape from a material mainly composed of copper. The lower portion in contact with the molten steel 2, that is, the inner surface of the mold 4 is coated with a Co-based alloy, a Ni alloy, or the like. Specifically, as shown in FIG. 1, the range A other than 200 to 300 mm from the upper end of the mold 4 to the lower side is plated with a thickness of 0.5 to 2.0 mm.

As shown in the plan view of FIG. 2, the electromagnetic stirrer 6 is a general one that has been conventionally used in a continuous casting apparatus, and turns the molten steel 2 clockwise (clockwise) or turns the molten steel 2 counterclockwise. (Counterclockwise). FIG. 2 is a development view in which the mold 4 is developed, and shows the flow of the molten steel 2 and the corrosion (erosion) state of the inner surface.
In the continuous casting apparatus 1, the molten steel 2 produced from a converter, secondary refining equipment, etc. is transported to the tundish 3 by a ladle, and the molten steel 2 in the transported ladle is poured into the tundish 3 and then immersed. While opening the nozzle 7 and stirring the molten steel 2 in the mold 4 in one direction by the electromagnetic stirring device 6, the molten steel 2 can be continuously cast. In this continuous casting apparatus, several molten steels 2 of the same steel type can be continuously cast by several charges, or molten steels 2 of different steel types can be continuously cast.

In the continuous casting apparatus 1, the tundish 3 can be moved by a moving device (not shown), and the tundish 3 can be exchanged.
Hereinafter, the continuous casting method of the present invention will be described in detail.
In the continuous casting method according to the present invention, when the molten steel 2 is supplied from the tundish 3 to the mold 4 again after the supply of the molten steel 2 from the tundish 3 to the mold 4 is cast again, the molten steel in the mold 4 is cast. Casting is performed after changing the molten metal level in No. 2 by 15 mm or more from the molten metal level before the supply of the molten steel 2 is stopped.

Specifically, first, the molten steel 2 in the tundish 3 is supplied to the mold 4, and the casting is started while the molten steel 2 is rotated to the right to stir the molten steel 2. Then, after several charges, when the steel type to be cast is changed or when the maintenance of the tundish 3 is performed, the immersion nozzle 7 is closed and the supply of the molten steel 2 from the tundish 3 to the mold 4 is stopped (casting is temporarily performed). Stop automatically).
After the casting is temporarily stopped, another molten steel 2 having a different steel type is inserted into the tundish 3, or the tundish 3 is replaced with a new one. Then, the immersion nozzle 7 is opened, the molten steel 2 supply from the tundish 3 to the mold 4 is resumed, and casting is performed again.

Hereinafter, for convenience of explanation, the process of restarting the molten steel 2 supply and performing casting may be referred to as a recasting process. In addition, the process of casting the molten steel 2 into the mold 4 before casting may be referred to as a pre-casting process.
As shown in FIG. 1, in the re-casting process, the molten steel supply amount and casting in the submerged nozzle 7 are set so that the molten metal level L2 in the re-casting process is 15 mm or more lower than the molten metal level L1 in the pre-casting process. Casting while controlling the speed. In the re-casting process, when the molten metal level is lowered from the previous casting process, the amount of the immersion nozzle 7 charged into the molten steel 2 is lowered corresponding to the lowered amount of the molten metal level.

Normally, during casting, powder is put on the molten steel 2 and the powder is interposed between the molten steel 2 (solidified shell) and the mold 4. In this embodiment, it is a standard for the molten metal surface level. The position is not on the upper surface of the powder but on the surface of the molten steel 2.
When casting as described above, as shown in FIG. 2, in the pre-casting process, the upper side of the inner surface of the mold 4 is easily corroded or eroded in the range B on one side in the left-right direction and on the upper side, The other portions are hardly eroded or eroded.
Further, in the re-casting process, the molten metal surface level L2 is lower than the molten metal surface level L1 of the pre-casting process. Is easily corroded or eroded, and the range B is in a state of being hardly corroded or eroded.

Therefore, by changing the portion where the molten steel 2 surface contacts the upper side of the inner surface of the mold 4, the corrosion or erosion on the upper surface of the inner surface of the mold 4 is dispersed, and as a result, the upper life of the mold 4 is extended. Can do.
Further, in this continuous casting method, in the recasting process, in addition to lowering the molten metal surface level L1 by 15 mm or more from the molten metal surface level L2 of the previous casting process, the stirring direction for electromagnetically stirring the molten steel 2 in the mold 4 is changed. Then, it is preferable to perform casting.
For example, during the pre-casting process, the molten steel 2 is turned to the right to stir the molten steel 2 (in the molten steel in FIG. 2, solid line), and during the re-casting process, the molten metal level L1 is set to the molten metal level in the pre-casting process. The molten steel 2 is lowered by 15 mm or more from the level L2, and the molten steel 2 having the lowered level is turned leftward to stir the molten steel 2 (inside the molten steel in FIG. 2, broken line).

As shown in FIG. 2, if the level of the molten steel is lowered and the stirring direction of the molten steel 2 is changed as described above, the recasting process is performed on the other side in the left-right direction on the upper side of the inner surface of the mold 4. The lowermost range D is easily corroded or eroded, and the other ranges are hardly corroded or eroded.
Therefore, it is possible to change the inner surface portion that is likely to hit the mold 4 by electromagnetic stirring, disperse the erosion on the upper surface of the inner surface of the mold 4, and extend the life of the upper surface of the mold 4.
In the re-casting process, the molten metal surface level is not changed, and the molten metal surface level L2 in the re-casting process and the molten metal surface level L1 in the pre-casting process may be the same, and only the stirring direction may be changed. That is, it is only necessary to reverse the stirring direction in the horizontal direction between the pre-casting process and the re-casting process.

In this case, in the re-casting process, the area F on the upper side of the inner side of the mold 4 on the other side in the left-right direction and the uppermost side is easily corroded or eroded, and the other areas are hardly corroded or eroded. Can do.
Next, in the re-casting process, it will be described in detail that the molten metal level is 15 mm or more than the molten metal level in the pre-casting process and the stirring direction is changed.
First, the inventor investigated the influence which the dent formed in the inner surface upper side gives to the cast slab when the inner surface upper side of the mold 4 is not plated.

That is, the inventor conducted an experiment in which continuous casting was repeatedly performed using the mold 4 without performing plating on the upper inner surface (portion other than the range A) of the mold 4, and as shown in FIG. The relationship between the occurrence rate of surface cracks and the maximum amount of dent (maximum wear amount) on the upper side of the inner surface of the mold 4 (other than the range A) was examined.
In addition, the surface crack for every steel type inspects the surface of the cast slab with an MT inspection device, and the surface of the slab (near the surface side inside the slab) has an opening of 5 mm or more. “Available” and those having an opening of less than 5 mm were regarded as “no defects”. Then, the occurrence rate of surface cracks was determined by dividing the total number of slabs inspected from the number of “defects”.

As shown in FIG. 3, as the maximum dent amount increases, the occurrence rate of surface cracks in the slab increases. When the maximum dent amount exceeds 0.3 mm or more, surface cracks occur in all steel types. It has occurred.
Therefore, in the mold 4 in which only the lower inner surface of the mold 4 is plated (the mold 4 in which the inner surface on the upper side of the mold 4 is not plated), when the dent formed on the upper surface of the mold 4 becomes 0.3 mm or more. It was found that the mold 4 reached the end of its life and that the mold 4 had to be replaced with a new one.

In addition, the inventor examined the distribution of dents on the upper surface of the inner surface of the mold 4 after repeated casting without performing plating on the upper inner surface (portion other than the range A). FIG. 4 summarizes the distribution of dents on the upper side of the inner surface.
As shown in FIG. 4, the range where the dent is 0.3 mm or more in the upper part of the inner surface where the molten steel surface of the molten steel 2 contacts is about 15 mm toward the casting direction. Further, the dents on the upper side of the inner surface are concentrated not on the entire upper side of the inner surface but on one side.
The reason why the dents concentrate on one side on the upper side of the inner surface is that when the molten steel 2 is electromagnetically stirred, the molten steel 2 accelerated in the horizontal direction hits the inner surface of the mold 4 and flows while changing the flow direction of the molten steel 2. It is considered that the portion where the accelerated molten steel 2 is likely to hit the mold 4 has been recessed intensively.

As described above, according to the results of FIG. 4, in the re-casting process, the portion where the molten steel 2 surface and the upper surface of the mold 4 are in contact with each other is changed by 15 mm or more in the width direction, or the direction of the stirring direction is changed. It turned out that it is good to change the part which the molten steel 2 accelerated by this hits.
Next, the inventor examined the timing for changing the level of the molten steel 2 during casting or changing the direction of the stirring direction. The result is shown in FIG.
FIG. 5 summarizes the time of changing the molten metal surface level and the time of changing the stirring direction, and the defect rejection rate of inclusions in the slab. In the magnetic stirrer 6, the magnetic flux density at the meniscus position (measured at a position 15 mm away from the outer surface of the copper plate) was 80 to 300 gauss, and the frequency was 1 to 4 Hz.

Case 1 is a case where the molten metal level is changed at the time of switching, and Case 2 is a case where the molten metal level is changed during casting without being changed at the time of switching. Case 3 has a stirring direction changed during casting, and case 4 has been cast without using electromagnetic stirring.
As shown in FIG. 5, when the hot water level is changed at the time of switching (case 1), the defect rejection rate is 0.1% or less and the rate of occurrence of very defective products is low, and the hot water level is changed during casting. In the case (Case 2), the defect rejection rate is 3.4%, which is a very high proportion of defective products.

Therefore, in the continuous casting method of the present invention, if the molten metal level is changed at the time of switching, the occurrence rate of defective products in the cast slab is low compared with the case where the molten metal level is changed during casting. Sometimes the hot water level was changed.
In addition, when the stirring direction is changed during casting (case 3), the defect rejection rate is 0.8%, and when the casting is performed without using electromagnetic stirring (case 4), the defect rejection rate is almost the same. The degree of occurrence of defects increases with the degree.
Therefore, if the hot water level is changed during casting, the entrainment of the mold bower introduced into the mold or the molten steel flow in the mold will change rapidly at the changed timing, and the incidence of defective products will increase. The stirring direction was changed at the time of switching, not during casting.

FIG. 6 summarizes an example in which casting was performed by the continuous casting method of the present invention and a comparative example in which casting was not performed by the continuous casting method of the present invention. In Examples and Comparative Examples, in the magnetic stirring device 6, the magnetic flux density at the meniscus position (measured at a position 15 mm away from the outer surface of the copper plate) was 80 to 300 gauss, and the frequency was 1 to 4 Hz.
The case 5 is obtained by producing a mold 4 in which only the lower inner surface of the mold 4 is plated, and continuously casting the mold 4 without changing the level of the molten metal surface.
The case 6 was produced by producing a mold 4 plated on the upper inner surface of the mold 4 in addition to the lower inner surface of the mold 4 and continuously casting the mold 4 without changing the level of the molten metal surface. It is.

Cases 7 to 9 are produced by casting the mold 4 in which only the lower inner surface of the mold 4 is plated, and changing the level of the molten metal or changing the stirring direction.
Specifically, the case 7 is obtained by continuously casting without changing the molten metal surface level in the pre-casting process and the re-casting process, and changing only the stirring direction, and the case 8 includes the pre-casting process, In the recasting process, the stirring direction is not changed and the casting surface level is changed continuously by 15 mm or more. The case 9 is changed in the stirring direction in the recasting process, and the precasting process and the recasting process. Casting was carried out by changing the hot water level in the process.

As shown in FIG. 6, when casting is performed with the stirring direction changed (case 7) or when the casting surface level is changed by 15 mm or more (case 8), the upper surface of the inner surface of the mold 4 is plated. The mold life was improved compared to the mold 4 produced.
That is, even if the upper side of the inner surface of the mold 4 is not plated, the life of the mold 4 can be improved by changing the stirring direction or changing the level of the hot water surface.
In particular, when the casting direction was changed while changing the stirring direction and the molten metal surface level was changed to 15 mm or more (Case 9), the mold life was 500 ch or more, and the life could be extended significantly.

  In addition, in the said embodiment, although the timing which stops supply of the molten steel 2 was said when changing the steel type to cast or when performing the maintenance of the tundish 3, it may be other than this. .

It is a conceptual diagram of a continuous casting apparatus. It is a figure which shows a corrosion (erosion) state in the inner surface upper side of a casting_mold | template. It is a figure which shows the relationship between the surface crack of a slab, and the amount of dents. It is a figure which shows distribution of a dent in the inner surface upper side of a casting_mold | template. It is a figure which shows the relationship between the change time of a hot_water | molten_metal surface level and a stirring direction, and a defect rejection rate. It is a figure which shows the mold lifetime in an Example and a comparative example. It is a figure which shows the relationship between the lifetime of a casting_mold | template, and the [Zn] density | concentration in molten steel.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Continuous casting apparatus 2 Molten steel 3 Tundish 4 Mold 5 Support roll 6 Electromagnetic stirring apparatus 7 Immersion nozzle

Claims (4)

A continuous casting method of supplying molten steel in a tundish to a mold and casting the supplied molten steel while electromagnetically stirring in one direction,
After the supply of molten steel from the tundish to the mold is stopped, when the molten steel is supplied again from the tundish to the mold and casting is performed, the molten metal level in the molten steel in the mold is the same as before the supply of the molten steel is stopped. A continuous casting method characterized in that casting is carried out after changing by 15 mm or more from the level of the molten metal. A continuous casting method of supplying molten steel in a tundish to a mold and casting the supplied molten steel while electromagnetically stirring in one direction,
After stopping the supply of molten steel from the tundish to the mold, when casting the molten steel from the tundish to the mold again, the casting is performed after changing the stirring direction for electromagnetically stirring the molten steel in the mold A continuous casting method characterized by that. A continuous casting method of supplying molten steel in a tundish to a mold and casting the supplied molten steel while electromagnetically stirring in one direction,
After the supply of molten steel from the tundish to the mold is stopped, when the molten steel is supplied again from the tundish to the mold and casting is performed, the molten metal level in the molten steel in the mold is the same as before the supply of the molten steel is stopped. A continuous casting method characterized in that casting is performed after changing the stirring direction in which the molten steel in the mold is electromagnetically stirred while being changed by 15 mm or more from the molten metal level.   The continuous casting method according to any one of claims 1 to 3, wherein the timing of stopping the supply of molten steel is when the type of steel to be cast is changed or when tundish maintenance is performed.